As long as you are not in direct work related contact, there is not an issue. Here they are.
It's normal to have company policies against it, because it causes issues in project teams, for 360 performance reviews etc.
Example: In your team of 3, your spouce does crappy work, who gets the blame? The last guy.
After a hard breakup, are you going to be doing great work as a team with your ex? When you get kids and one ends up with custody, will it still be great?
It only gets worse if one partner is supervisor, and nobody can complain about the other, but even best case, can the boss fire/reprimand one partner without having issues with the other?
That is why it is in general best if one partner moves to other divisions etc.